The present invention relates to a system for measuring the relative position of two objects by using individual code marks, each consisting of several code mark elements, to absolutely identify individual reference marks provided along the graduation of a measuring scale.
Length or angle measuring instruments are used to measure the relative position of two movable objects. One type of such instrumentation includes a scale comprising an incremental graduation attached to the first object and a scanning device, capable of reading the scale, attached to the second object. Such measuring instruments are frequently used on machine tools to measure the relative position of the slide unit with respect to the machine bed.
U.S. Pat. No. 3,982,106 discloses a length or angle measuring system in which reference marks are provided on a separate track beside the incremental graduation marks. The absolute values of these reference marks are determined from the different spacings between the individual reference marks. These spacings are determined by scanning the incremental graduation. Two reference marks must be scanned to determine the absolute position of any given reference mark. This process can be complicated and time consuming if the two desired reference marks lie far apart. Moreover, if an error occurs in counting the increments between two reference marks, the determination of the spacing between the reference marks will be incorrect. This may result in a false determination of the absolute value for the reference mark.
U.S. Pat. No. 4,403,859 discloses an incremental length or angle measuring arrangement in which reference marks, with different line group distributions, are provided on a scale beside the graduation marks. The individual reference marks are aligned to be scanned by respective scanning fields of a scanning unit, and each scanning field is unambiguously allocated to a respective reference mark. This arrangement is relatively complicated since, in order to clearly identify the individual reference marks, the line group distributions must differ as strongly as possible from one another. Furthermore, an identical scanning field of the scanning unit is required for each reference mark.
U.S. Pat. No. 4,519,140 discloses an incremental length or angle measuring arrangement in which identical reference marks are provided on a separate track beside the incremental graduation. In order to identify the identical reference marks, a serial code mark on a separate track is allocated for each reference mark. These code marks consist in each case of code mark elements which represent a particular binary state. The other binary state is represented by the absence of code mark elements, i.e. gaps. The entire code mark, in the form of code mark elements and gaps, is serially scanned to obtain the sequence of binary states. This sequence yields the entire code information for the identification of the associated reference mark.
In such measuring systems the absolute values of the reference marks are generally referenced to the graduation zero point of the incremental graduation of the scale. Since the spacing of the reference marks with respect to one another is known, the reference marks can be used for checking the accuracy of the graduation increment count. However, in order to check the accuracy of the graduation increment count between the reference marks the code mark elements must be used. These elements are serially allocated to the individual reference marks and lie between the reference marks. The absence and presence of the code mark elements represent the binary state of the code mark. The binary representation of some code marks may result in the absence of several adjacent code mark elements, i.e. gaps. These gaps may present a difficulty if they are relatively large, since without code mark elements or reference marks it is not possible to check the accuracy of the graduation increment count.